The present disclosure relates to a fixing device for fixing a toner image on a recording medium, such as a sheet or a film, and an image forming apparatus provided with the fixing device.
An electrographic image forming apparatus, such as a copying machine or a printer, is provided with a fixing device for fixing a toner image on a recording medium, such as a sheet or a film. For the fixing device, a heat fixing manner is generally applied to fuse a toner (a developer) by heating and to fix it onto the sheet. The fixing device with the heat fixing manner is provided with a first rotating member (e.g. a fixing roller) heated by a heat source and a second rotating member (e.g. a pressing roller) pressed against the first rotating member so that a fixing nip is formed between the first and second rotating members.
As a system for the aforementioned rotating members, a heat roller system having each roller as a rotating member is most often applied from a point of view of thermal efficiency and safety. On the other hand, in order to satisfy requests, such as reduction of a warm-up time and saving energy, a belt system having at least one rotating member as a belt has received attention in recent years. As the heat source for heating the aforementioned rotating member, a halogen lamp is generally utilized, but there are many products utilizing a ceramic heater or an induction heating (IH) coil to enable quick-heating and high-efficiency heating.
Incidentally, in the aforementioned fixing nip, the toner image is fixed onto the various recording mediums with different widths according to user's needs. When the toner image is fixed on a recording medium with a first size in the fixing nip, in one region on the first rotating member which the first size recording medium passes (hereinafter, called as “a first sheet passing region”), the first size recording medium and non-fixed toner are heated, thereby consuming heat. On the other hand, in another region outside the first sheet passing region which another recording medium with a second size having a broader width than the first size recording medium passes (hereinafter, called as “a first non-sheet passing region”), there is no consumed heat. Therefore, the temperature of the first non-sheet passing region on the first rotating member is higher than the temperature of the first sheet passing region, and then, a temperature distribution of the first rotating member becomes non-uniform.
Such non-uniformity of the temperature distribution is especially intensified when the toner image is continuously fixed on the first size recording medium (e.g. a standard sized sheet called B5). In a situation of such an non-uniformit of temperature distribution, when the toner image is fixed on the second size recording medium (e.g. a standard sized sheet called A4), a problem of causing a fixation irregularity and a wrinkle onto the second size recording medium occurs. In addition, there is another problem called as “hot offset” that the toner melts on a corresponding part to the first non-sheet passing region too much and adheres to the surface of the first rotating member, and consequently, dirties the surface of the following recording medium entering the fixing nip. Particularly, in a case of using the heat roller system, when the temperature of the first non-sheet passing region is heightened too much in comparison with the first sheet passing region, the first non-sheet passing region and first sheet passing region are different from each other in thermal expansion. Accordingly, a further problem is caused that the roller composing the first rotating member is distorted so that this roller deteriorates. Furthermore, when the ceramic heater is applied as the heat source, there is serious disadvantage of making the heater broken due to the non-uniformity of the temperature distribution.
Then, in order to prevent the non-uniformity of the temperature distribution, some fixing devices with IH system may apply a configuration for controlling magnetic flux distribution or properly using a plurality of different IH coils. However, when such a configuration is applied, it is feared that the controlling system for the fixing device is complicated to bring an increase in cost.
Thereupon, in order to prevent the non-uniformity of the temperature distribution without complicating the controlling system, the configurations of cooling the rotating member, such as the fixing roller, by an airflow blown from a fan are known as follows.
As an example, there is a configuration that a fan selectively blows an airflow toward a first rotating member of a first non-sheet passing region to prevent an increase in the temperature of the first non-sheet passing region. However, in this configuration, it is feared that a part of the airflow blown toward the first non-sheet passing region is fed to a first sheet passing region to decrease the temperature of the first sheet passing region.
As another example, there is another configuration that a plurality of airflow duct windows connected with a fan are provided in an axial direction of a first rotating member to adjust an opening angle of the airflow duct window by an airflow duct adjusting board. However, in this configuration, because the airflow duct adjusting board must be added, it is feared of complicating the structure of components and increasing cost. In addition, in a case of applying the so-called “external capsuled IH”, in which an IH coil is placed outside the first rotating member, it is very difficult to secure a space for placing the airflow duct adjusting board around the first rotating member.
As a further example, there is a further configuration that a fan is inclined against a width direction of a first rotating member to minimize a cooling airflow going round to a first sheet passing region. However, in the further configuration, it is feared that the inclination of the fan causes no cooling airflow to a part of a first non-sheet passing region, and then, that reduction of cooling area causes reduction of cooling effect.
As a furthermore example, there is a furthermore configuration that a cooling airflow from a fan is blown toward a second rotating member to cool the second rotating member. However, when the cooling airflow is fed to the second rotating member and only heat transfer on the second rotating member cools a first rotating member, cooling efficiency of the first rotating member is lower than a case of cooling the first rotating member directly. Therefore, there is a problem that the fan must be enlarged in order to sufficiently cool the first rotating member.